Wavelength-division multiplexing (WDM) has enabled a dramatic increase in the transmission capacity of fiber-optic systems. WDM combines a plurality of sub-beams at different wavelength bands for propagation through an optical fiber as a multiplexed beam. As each sub-beam carries a signal, multiple signals can be transmitted simultaneously. Wavelength switches are used to route the individual sub-beams, and the signals they carry, along particular paths.
With reference to FIGS. 1A-1B, in a prior-art wavelength switch 100 disclosed in U.S. Pat. No. 6,707,959 to Ducellier, et al., which is owned by the assignee of the present invention and is incorporated herein by reference, a multiplexed input beam is launched from an input port in a front-end unit 110 towards a lensing element 120 with optical power, for example, a spherical mirror with positive optical power. The input beam is redirected from the lensing element 120 to wavelength-dispersing element 130, for example, a reflecting diffraction grating, which disperses the input beam into a plurality of sub-beams at different wavelength bands. The sub-beams are then redirected from the wavelength-dispersing element 130, via the lensing element 120, to a switching stage in a back-end unit 140. The switching stage includes an actuation array of reflecting elements, for example, a microelectromechanical system (MEMS) mirror array, which routes each sub-beam to a selected one of a plurality of output ports in the front-end unit 110. Each sub-beam is associated with a reflecting element of the actuation array, and the associated reflecting element can be tilted about an axis to route the sub-beam along a path leading to the selected output port. The sub-beams are redirected from the switching stage in the back-end unit 140, via the lensing element 120, back to the wavelength-dispersing element 130, which combines those sub-beams that are routed to a same output port. The sub-beams are then redirected from the wavelength-dispersing element 130, via the lensing element 120, to the selected output ports in the front-end unit 110, which output the sub-beams.
In such a wavelength switch including a single actuation array of reflecting elements of a single switching stage, the number of output ports is limited by the angular range through which the individual reflecting elements of the actuation array can be tilted. Currently, the maximum number of output ports is about 11. However, to demultiplex a larger number of signals, a larger number of output ports is desired.
One approach to achieving a larger number of output ports involves combining several wavelength switches, each of which includes a single actuation array of reflecting elements, to form a switching cascade, as disclosed in U.S. Pat. No. 6,657,770 to Marom, et al., for example. The output ports of one wavelength switch of a first switching stage are coupled to the input ports of a plurality of wavelength switches of a second switching stage. The maximum number of output ports is, thereby, squared. With reference to FIG. 2, the combination of one 1×3 wavelength switch 210, which has one input port 211 and three output ports 212-214, with three 1×8 wavelength switches 220, 230, and 240, which each have one input port 221, 231, and 241 and eight output ports 222, 232, and 242, forms a 1×24 switching cascade 200.
However, in this approach, sub-beams output from the wavelength switch of the first switching stage must be coupled back into fibers before being launched into the plurality of wavelength switches of the second switching stage, which entails high insertion losses. Thus, a wavelength switch incorporating two switching stages, a first switching stage including a single actuation array of reflecting elements and a second switching stage including a plurality of actuation arrays of reflecting elements, in a single device is desired.
Wavelength cross-connects including two switching stages in a single device are disclosed in U.S. Pat. No. 6,694,073 to Golub, et al., U.S. Pat. No. 6,870,982 to Maheshwari, U.S. Pat. No. 6,922,500 to Huang, et al., U.S. Pat. No. 7,088,882 to Ducellier, et al., and U.S. Patent Application No. 2005/0117837 to Cerato, for example. In such wavelength cross-connects, an actuation array of reflecting elements of a first switching stage routes sub-beams to a plurality of actuation arrays of reflecting elements of a second switching stage. However, each second-stage actuation array routes sub-beams to a single output port associated with that actuation array. Thus, the disclosed wavelength cross-connects do not fulfill the goal of achieving a larger number of output ports in a single device.
An object of the present invention is to overcome the shortcomings of the prior art by providing a wavelength switch including two switching stages. A single actuation array of reflecting elements of a first switching stage routes sub-beams to a plurality of actuation arrays of reflecting elements of a second switching stage. Each second-stage actuation array routes sub-beams to a group of output ports associated with that second-stage actuation array. The sub-beams are redirected from the first switching stage to the second switching stage by a reflecting relay assembly, without being combined or coupled into fibers, allowing the number of output ports to be increased, without incurring additional insertion losses.